.\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
.\"
.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
.\" Distributed under GPL
.\" %%%LICENSE_END
.\"
.\" Based on glibc infopages
.\"
.\" Corrections by aeb
.\"
.TH NAN 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
nan, nanf, nanl \- return 'Not a Number'
.SH SYNOPSIS
.nf
.B #include <math.h>
.PP
.BI "double nan(const char *" tagp );
.BI "float nanf(const char *" tagp );
.BI "long double nanl(const char *" tagp );
.fi
.PP
Link with \fI\-lm\fP.
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR nan (),
.BR nanf (),
.BR nanl ():
.nf
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
These functions return a representation (determined by
.IR tagp )
of a quiet NaN.
If the implementation does not support
quiet NaNs, these functions return zero.
.PP
The call
.I nan("char\-sequence")
is equivalent to:
.PP
.in +4n
.EX
strtod("NAN(char\-sequence)", NULL);
.EE
.in
.PP
Similarly, calls to
.BR nanf ()
and
.BR nanl ()
are equivalent to analogous calls to
.BR strtof (3)
and
.BR strtold (3).
.PP
The argument
.I tagp
is used in an unspecified manner.
On IEEE 754 systems, there are many representations of NaN, and
.I tagp
selects one.
On other systems it may do nothing.
.SH VERSIONS
These functions first appeared in glibc in version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR nan (),
.BR nanf (),
.BR nanl ()
T}	Thread safety	MT-Safe locale
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
C99, POSIX.1-2001, POSIX.1-2008.
See also IEC 559 and the appendix with
recommended functions in IEEE 754/IEEE 854.
.SH SEE ALSO
.BR isnan (3),
.BR strtod (3),
.BR math_error (7)
